This invention relates to a thermosetting resin composition excellent in curing properties to provide a cured product good in properties, particularly dielectric properties, and a prepreg and an electrical laminate excellent in electrical properties and mechanical properties obtained from said resin composition.
Since phenol resins have good balance in various properties such as electrical properties, mechanical properties, heat resistance, chemical resistance, etc., and are non-expensive, these resins have widely been used in electrical and electronic fields for public uses. Further, epoxy resins which have improved defects of the phenol resins such as the generation of by-products at the time of reaction and badness in adhesiveness have also been used in very wide fields because of excellence in molding properties (small in cure shrinkage).
Phenol resins and epoxy resins, however, have a defect in that a long period of time is necessary for curing due to polycondensation or ion polymerization curing reaction. On the other hand, unsaturated polyester resins are excellent in curing properties. Since the curing reaction of unsaturated polyester resins is carried out by radical polymerization, curing at room temperature becomes possible by properly selecting a catalyst system. But, since a large number of polar groups (carbonyl groups) are contained in the resin skeleton, the resulting cured product easily absorbs moisture. Thus, the unsaturated polyester resins have a defect in that various properties are deteriorated after moisture absorption.
On the other hand, electrical laminates have heretofore been composed of substrates such as paper, glass cloth or the like and a thermosetting resin such as a phenolic resin, epoxy resin, melamine resin, unsaturated polyester resin or the like. As a method for producing them, there is employed a method comprising impregnating the substrate with a liquid of said resin, then laminating the substrates, and curing the resin.
In the case of phenolic resins, epoxy resins and melamine resins, a solution thereof in a suitable solvent is prepared, infiltrated into substrates, and dried by heating to obtain prepregs in B-stage, which are then heated and pressed to obtain a laminate.
Therefore, the solvent which has become useless volatilizes during the production, so that a large-scale equipment is required for recovering or disposing of the solvent. Furthermore, when the prepregs in B-stage are laminated and heated and pressed to be cured, a high-pressure press is needed. For these reasons and so on, the above method is poor in productivity.
In recent years, there has been proposed a production method in which substrates impregnated with an unsaturated polyester resin are laminated and cured under no pressure. This method has a high productivity because of the good curing property of the resin, but is disadvantageous in that when the proportion of styrene is increased in order to improve the electrical characteristics, the mechanical properties of the resulting laminate are deteriorated, for example, the laminate becomes hard and fragile. When the proportion of styrene is reduced, the viscosity is increased, resulting in insufficient infiltration into substrates, or loss of the superiority in electrical characteristics.